The invention relates to novel paclitaxel derivatives useful in the treatment of cancer. More specifically, the invention relates to paclitaxel derivatives obtained by substitutions at the C2xe2x80x2, C3xe2x80x2 and/or C7 positions. The invention also provides pharmaceutical compositions containing the said derivatives and useful in the treatment of cancer affecting the lung, breast, ovary, cervix, etc. Further, the invention provides methods for the preparation of such novel paclitaxel derivatives.
Paclitaxel is a diterpenoid taxane derivative from the bark of Pacific yew, Taxus brevifolia. It was first discovered in 1971 following investigation of bark extracts from the European yew tree, Taxus baccata. Paclitaxel was randomly isolated and found to exhibit cytotoxic properties. The basic structure of paclitaxel is as depicted in FIG. 1. The central backbone unit of paclitaxel contains Baccatin III. Although Paclitaxel is structurally similar to vinca alkaloids, its mechanism of action is unique. It promotes the microtubule assembly by enhancing the action of tubulin dimers, stabilizing existing microtubules, and inhibiting their disassembly. Paclitaxel was originally approved for the treatment of refractory ovarian cancer and was subsequently approved for the use in breast carcinoma. It has also been used alone or as an adjunct therapy in other malignancies such as advanced head and neck cancer, metastatic breast cancer, non-small-cell lung cancer, acute leukemia, and melanoma.
Since paclitaxel shows great promise as a chemotherapeutic agent, several researchers have spent substantial time in developing potent derivatives of paclitaxel. Attachment of the C-13 side chain of paclitaxel to other naturally occurring taxanes and taxoids, modification of the diterpene moiety of paclitaxel at various centers such as 7-hydroxyl group, 10-hydroxyl group and modifications at C-2, 9, 19, 6 and 4 positions have been well documented. Other approaches have been modification at the C-13 side chain of paclitaxel, and in recent times, the development of water-soluble paclitaxel prodrugs.
Modifications at the diterpene moiety as well as the C-13 side chain of paclitaxel has resulted in several derivatives of paclitaxel which have shown varying cytotoxic activities on human tumor cell lines in in vitro assays. In the diterpene moiety of paclitaxel, the 4-acetyl group and the 2-benzoyloxy moiety of paclitaxel are essential structures for cytotoxicity. Modifications of the A-ring and B-ring did not affect microtubule binding in a significant way. The acetoxy group at C-10 can be deleted while esterification of the C-10 hydroxyl group with a variety of acids provided active compounds. Of all the modifications made on the diterpene portion of the molecule, most of the initial structural changes have involved the derivatisation of the 7-hydroxyl group. The 7-hydroxyl group can be modified or epimerized without significant loss of bioactivity. Examples include 7-acetyltaxol, 7-benzoyltaxol, 7-glutaryl derivatives, and C-7 amino acid esters. Modifications at C-19 may be tolerated without significant loss of bioactivity.
Most of the C-13 simplified paclitaxel derivatives and derivatives of different stereochemistry demonstrated reduced activity in comparison to paclitaxel. The 2xe2x80x2-hydroxyl and the 3xe2x80x2-benzamido group are not essential for bioactivity, but are important for strong microtubule binding and cytotoxicity. Formation of ethers at 2xe2x80x2-hydroxyl group, such as methyl ether and ter-butyldimethylsilyl ether, reduced cytotoxicity. Acetylation of the 2xe2x80x2-hydroxyl group (2xe2x80x2-acetyltaxol) also leads to loss of activity. Replacement of 2xe2x80x2-hydroxyl group by fluorine was also found to significantly reduce the cytotoxicity.
A problem associated with the systemic administration of Paclitaxel is its low solubility in most pharmaceutically acceptable solvents. Most formulations used clinically contain Cremophor EL (polyethoxylated castor oil) and ethanol as excipients, which may cause hypersensitivity reactions. This can be prevented by pre-medication with certain antihistaminic drugs. To eliminate this vehicle and possibly reduce the dose and hence toxicity of paclitaxel, several approaches of drug targeting are currently being worked upon. Much work is being done on formulation of paclitaxel in liposomes of various compositions. Paclitaxel-liposomes retain the growth-inhibitory activity of the free drug in vitro against a variety of tumor cell lines. In mice, paclitaxel-liposomes were well-tolerated when given in bolus doses (Sharma A., et al, Pharm Res: 11, 1994). The Maximum Tolerated Dose (MTD) was  greater than 200 mg/kg which exceeded that of free paclitaxel (MTD of 30 mg/kg by iv or 50 mg/kg by ip administration). Free paclitaxel administered in the Cremophor vehicle was toxic at doses  greater than 30 mg/kg, as was the equivalent volume of vehicle without drug. However, the low stability of these formulations is still an area of concern. Some of these formulations are physically and chemically stable for only 2 months at 4 degrees C., or for 1 month at 20 degrees C.
Yet another approach of tumor targeting is the administration of paclitaxel-loaded poly (lactic-co-glycolic acid) microspheres containing isopropyl myristate (Paclitaxel-IPM-PLGA-MS). After administration of the drug saline solution, paclitaxel disappears rapidly from plasma and distributes extensively in various tissues. The tissue levels of paclitaxel in the lung were found to be higher than those in plasma but relatively lower than those in kidneys, bile, and liver (Sato H. et al, Biol Pharm Bull: 19, 1996). The biodistribution of paclitaxel after administration of Paclitaxel-IPM-PLGA-MS (3 mg paclitaxel/kg), on the other hand, was altered significantly from the control (paclitaxel solution) group. Paclitaxel concentrations in the lung were increased significantly with the microsphere group. It was also noticed that the paclitaxel levels in the lung were maintained at relatively high levels. Thus, it may be possible to use Paclitaxel-IPM-PLGA-MS for targeted delivery of paclitaxel to the lung. It however remains to be seen if this strategy can be used for targeting paclitaxel to tumors other than lung.
Enzyme-activatable prodrugs in conjunction with antibody-enzyme fusion proteins offer an alternative to enhance the anti-tumor efficacy of antibodies and reduce the toxic side effects of conventional chemotherapeutics. Cephalosporins have proven to be highly versatile triggers for the enzymatic activation of such prodrugs. A cephem prodrug of paclitaxel (PROTAX) was synthesized by substituting the C-3xe2x80x2 position of cephalothin with 2xe2x80x2-(gamma-aminobutyryl) paclitaxel (Rodrigues M. L., et al, Chem Biol: 2, 1995). Hydrolysis of PROTAX by beta-lactamase rapidly releases 2xe2x80x2-(gamma-aminobutyryl) which yields paclitaxel following intramolecular displacement. PROTAX is inactive in a microtubule assembly assay in vitro but has similar activity to paclitaxel following prolonged activation with beta-lactamase. PROTAX is approximately 10-fold less toxic than paclitaxel against SK-BR-3 breast tumor cells in vitro but has activity approaching that of paclitaxel following prolonged activation with a fusion protein comprising beta-lactamase fused to a tumor-targeting antibody fragment. Tubulin polymerization activity is abolished and cytotoxicity is reduced in the PROTAX prodrug compared to paclitaxel.
Li. C., et al have recently reported in Cancer Research, 1998, p.2404, that a water soluble poly (L-glutamic acid)-paclitaxel (PG-TXL) conjugate produces striking antitumor effects with diminished toxicity. A single intravenous injection of PG-TXL of 40 mg paclitaxel/kg resulted in the disappearance of an established implanted 13762F mammary adenocarcinoma. PG-TXL has a prolonged half-life in plasma and a greater uptake in tumor as compared with paclitaxel.
Abraham E. Mathew et. al, J.Med.Chem, p.145(1992) reports the synthesis and evaluation of water soluble 2xe2x80x2 and 7-amino acid derivatives of paclitaxel with antitumor activity.
Renee Paradis and Michel Page, Anticancer Research, 1998 p.2711 reports new active paclitaxel amino acid derivatives with improved water solubility.
JP 09235225 discloses the C2xe2x80x2-(un)protected amino acid residue and peptide residue conjugates of paclitaxel which have shown in vivo efficacy in guinea pigs implanted with melanoma cells.
U.S. Pat. No. 5,719,265 deals with the synthesis of N-protected amino acid derivatives at 2xe2x80x27-position, 2xe2x80x2-position amino acid is cleaved and then the 7-position amino acid is deprotected which improves the solubility and their use as anti tumor agent. EP 0747385A1 discloses the synthesis of paclitaxel analogues modification at 2xe2x80x2-position and 3xe2x80x2-amino position as different benzoyl derivatives and their use as antitumor agents.
Indian Journal of Chemistry, Vol.38B, p.1194 (1999) deals with the anticancer activities of C2 modified paclitaxel analogues.
U.S. Pat. No. 5,917,062 teaches the utilization of new C7 alkyl carbonate or alkyl carbonyl derivative of 10-Deacetyl Baccatin III for preparation of semisynthetic paclitaxel and its analogues.
U.S. Pat. No. 6,017,935 provides C7-substituted sulphur analogues of paclitaxel having anti-tumor activity, pharmaceutical formulations and methods for the preparation thereof.
Thus, several workers have developed different analogues of paclitaxel. Still, there is a need for analogues that alleviate toxic side effects and provide improved dose therapy. It is towards this goal that the applicants have worked and developed novel paclitaxel analogues.
The main object of the invention is to provide novel paclitaxel derivatives obtained by modifying effecting modifications C2xe2x80x2, C3xe2x80x2 and/or C7 positions of paclitaxel, said derivatives being useful in the treatment of cancer.
Another object is to provide pharmaceutical compositions containing the said novel paclitaxel analogues, useful in inhibiting tumor growth and, particularly, for inhibiting the growth of cancer of lung, breast, ovary, cervix or leukemia.
It is also an object of the invention to provide pharmaceutical compositions in improved dosage so that the toxic and adverse reactions associated with paclitaxel derivatives are reduced.
Yet another object of the invention is to reduce/or eliminate the hypersensitivity reactions caused by Cremophor EL.
Still another object is to provide a methods for the preparation of novel paclitaxel derivatives modified at C2xe2x80x2, C3xe2x80x2 and/or C7positions.
The invention provides novel paclitaxel derivatives obtained by effecting modifications at C2xe2x80x2, C3xe2x80x2 and/or C7 positions of paclitaxel, said derivatives being useful in inhibiting the growth of cancer of lung, breast, ovary, cervix or leukemia. Further, the invention provides pharmaceutical compositions employing the novel paclitaxel derivatives of the invention. In addition, the invention prescribes so that, the toxic and adverse reactions commonly observed with paclitaxel are reduced. Further, the invention provides pharmaceutical compositions employing excipients, which reduce or eliminate the hypersensitivity reactions generally observed by the use of Cremophor EL. The invention also teaches methods for the synthesis of the paclitaxel analogues of the invention are obtained primarily by sulphonation, acylation, benzoylation and/or condensation reactions.